In an OFDMA (Orthogonal Frequency Division Multiple Access) based system, the system frequency-time resources are often divided into small resource units which can be assigned to multiple different devices for transmission purposes. The smallest resource unit usually occupies one tone in the frequency domain and one OFDM-symbol in the time domain. Which tones devices in a communications system use for transmission purposes can have a significant effect on interference, e.g., interference caused by In-phase and Quadrature phase signal imbalance.
In-phase (I) and Quadrature phase (Q) signal imbalance, commonly referred to as IQ imbalance, occurs when the In-phase and Quadrature components are not fully symmetric, e.g., either in phase or in amplitude. IQ imbalance typically occurs both at the transmitter and at the receiver. In the case of single-tone transmissions by a transmitter, the IQ imbalance manifests itself as an image tone at frequency fc−f when an intended signal is sent at frequency fc+f by a transmitter, where fC is the frequency of the carrier signal used to modulate the baseband tone f. The image tone at frequency fc−f is an interference signal generated by the transmitter transmitting on tone fc+f. In other words, part of the energy transmitted on tone fc+f leaks to tone fc−f at the receiver. Due to the existence of this leakage, signaling or the traffic transmission in the system using frequency (fc−f), potentially transmitted from another communication device, can be corrupted. Although the leakage caused by the image tone may appear at a strength which is considerably lower than the signal on the original tone (fc+f), it can still be of considerable strength when compared to other transmissions on tone (fc−f), especially when the transmission on tone fc−f comes from another transmitter. If the problems caused by the image tone are left unmitigated, the achievable reliability of decoding/detection of signals may, and often is, reduced at a device trying to receive data on the image tone frequency. As we have discussed, this effect is more pronounced when the two baseband frequencies, e.g., (f, and −f) are simultaneously used by two different devices which are transmitting on the corresponding passband frequencies (fc+f and fc−f).
Consider a communications network 100 illustrated in FIG. 1. The network 100 includes a number of mobile wireless terminals including WT 1 102, WT 2 104, WT 3 106, WT 4 108 and WT 5 110. The wireless terminals 102 and 104 have an established communications link (WT 1 102 to WT 2 104). WT 1 102 transmits information to WT 2 104 using a frequency fc−f. However there is another simultaneous communications session going on where WT 4 108 transmits information to WT 3 106 using frequency fc+f over another communications link (WT 4 108 to WT 3 106). The WT 4 108 is physically in closer proximity of WT 2 104 than the WT 1 102. In such a scenario, the image frequency of fc+f appears at “fc−f” (at the receiving WT 2 104) and may strongly damage the information signal detection capability of WT 2 104. Furthermore, if WT 4 108 is much closer to WT 2 104 than WT 1 102 as shown in FIG. 1, e.g., with the pathloss from WT 4 108 to WT 1 102 being lower, the received signal due to the image tone generated by WT 4 108 can be comparable in strength or stronger than the information signal from WT 1 102 transmitted to WT 2 104 on the same tone (fc−f).
The interference caused by the image tones often results in a degradation of received signal quality at a receiver device WT 2 104, as the signals at any given frequency can be hidden under interference signals due to the existence of image tones from signals transmitted by unintended transmitters. In a system with a centralized controller, such as a cellular system where a base station or system controller controls tone assignments, interference can be mitigated by controlling the transmit power of different signals at different tones by taking into consideration the interference which will be caused by the assignment of tones to particular devices in the overall system.
In an ad hoc network, a centralized controller which controls tone assignments and transmit power in the system is normally lacking and the problem of interference due to the generation of image tones can be significant. Thus, it should be appreciated that the problem of interference caused by I/Q imbalance can be more damaging in an ad hoc network due to the arbitrary location of wireless terminals and lack of the central coordination device such as a base station.
In view of the above, it should be appreciated that there is a need for methods and apparatus that allow for the determination of what communications resources, e.g., tones, should be used that would reduce, avoid or mitigate the problem of dynamic range reduction at receivers due to interference caused by the generation of image tones. It would be desirable if the methods and/or apparatus were suitable for use in ad hoc networks where a centralized controller is not available for controlling tone allocation to different devices in the system.